1. Field of the Invention
The present invention relates generally to plug-type valves and more specifically, to an improved plug-type valve in which the vertical motion of the plug between the open position of the valve and the closed position of the valve is unidirectional and wherein the vertical motion of the plug between the closed position of the valve and the open position of the valve is also unidirectional.
2. Prior Art
Plug-type valves are well-known in the art. Often such plug-type valves are implemented with slips and seals in a seat and reseat configuration. The slips and seals provide means for blocking the flow through the valve at both the inlet and outlet ports when the valve is in the closed position. This permits maintenance personnel to bleed the valve body with the valve in its closed position with a minimum expenditure of time and labor costs. Typically, the plug and slips are interconnected so that in opening the valve from its fully closed position, the first motion of the plug is vertical in one direction which has the effect of retracting the slips from the inside surface of the valve body in order to prevent scoring of the valve seals in the subsequent motion when the valve is turned to place the flow passage to be in line with the inlet and outlet. However, in conventional plug-type valves which employ slips and seals as described above, after the flow passage has been aligned with the inlet and outlet, vertical motion of the plug is then again required in a direction opposite to that of the initial vertical motion which retracted the slips. This second oppositely directed vertical motion in effect reseats the valve by placing the flow passage in the plug in intimate contact with the inlet and outlet respectively.
Although such seat and reseat valves have had good commercial success, the aforementioned motion of the plug in opening or closing the valve, namely, vertical motion in a first direction, the 90 degrees rotation and then vertical motion in a second direction opposite to the first, generally increases the cost of the valve because of the complexity of the operator mechanism to which the plug is connected. The operator mechanism is complex because it has two stem sets to transform the aforementioned vertical motion, rotational motion and oppositely directed vertical motion into a undirectional motion in a hand operator or automatic operator. Consequently, the cost of manufacture, maintenance and repair of such a complex operator is higher than it would be if it were possible to simplify the motion of the plug between the valves open and closed positions. For example, if it were possible to design a plug-type valve in which the motion of the plug were vertical, rotational and vertical in opening and closing, wherein both vertical portions of the operation sequence were in the same direction for opening the valve and in the same direction for closing the valve, then the operating mechanism to which the plug trunnion were attached would be significantly simpler. The operator would also be less costly to manufacture, maintain and repair thereby reducing the overall costs for the valve while still providing a double block and bleed feature of the prior art valves described above as well as the reseat feature which allows the slips to be changed without draining the line.